Abstract: This case study describes how F.L.Smidth A/S, a manufacturer of large processing plants for cement production, has applied the principles of mass customisation in the area of highly complex, custom engineered products. The company has based its sales process on a configuration system to achieve a more efficient sales and engineering process. The implementation of the configuration system was accompanied by a radical redefinition of the modular structure of the company|s product architectures. The project was carried out in cooperation with the Centre for Product Modelling (CPM) at the Technical University of Denmark and illustrates the methods developed at the CPM for the construction of configuration systems. The case analyses the implementation process of the configuration system and documents the results attained after implementation.